1. Field of the Invention
The invention concerns a terminal for a bidirectional synchronous digital hierarchy (SDH) link with automatic protection switching conveying multiplexed data. A terminal of this kind is connected to another like terminal by two transmission lines for sending to the other terminal and for receiving from the other terminal, respectively. It can be used for underwater or transmission on a fiber optic cable. The synchronous digital hierarchy (SDH) is generally used for transmitting information over a fiber optic land line. Submarine transmission requires the multiplexing of a plurality of SDH multiplexes and service signals specific to submarine transmission. Each end of a submarine link generally has two SDH receive terminals, two SDH send terminals and a submarine link terminal equipment.
2. Description of the Prior Art
Each SDH receive terminal receives a plurality of SDH multiplexes from land lines and multiplexes them into the form of two identical higher order SDH multiplexes that it supplies to the submarine link terminal equipment. Each SDH send terminal receives two identical SDH multiplexes supplied by the submarine link terminal equipment. It partially demultiplexes them and supplies a plurality of lower order SDH multiplexes to land lines.
If the SDH send terminal is no longer receiving one of the two multiplexes that the submarine link terminal equipment should supply to it, it supplies a link far end failure signal specific to the multiplex concerned to enable diagnosis of a fault affecting the resources at the other end of the link. This signal is a far end remote failure (FERF) signal, for example. The SDH standard provide bits in each frame of 16th order SDH multiplexes for conveying this signal. This signal assumes the value 110 in the event of a fault.
Each submarine link terminal equipment includes:
two send devices, PA1 a switch for selecting one of the two multiplexes provided by the respective send devices and sending it in optical form over a submarine link, and PA1 a copier device receiving a multiplex supplied by a submarine transmission line in optical form and reconstituting it in the form of two identical multiplexes supplied to the respective SDH send terminals (the copier device preferably reconstitutes two optical signals, but could equally well reconstitute two electrical signals). PA1 two send devices each receiving n first and n second SDH multiplexes, where n is greater than or equal to 1, each of said SDH multiplexes respectively conveying n first and n second far end remote failure signals, each send device including: PA1 an automatic protection switch for transmitting to said first line one of the two multiplexes respectively supplied by said two send devices, PA1 two receive devices each having: PA1 a logic device for receiving the 4n far end remote failure signals supplied by each of said two receive devices and deducing therefrom an automatic protection switch control signal.
A device that is known for controlling the automatic protection switch of the submarine link terminal equipment includes SDH send terminals modified compared to the SDH terminals available off the shelf. Each submarine link terminal equipment being connected to a plurality of SDH send terminals, each of which produces an FERF signal, it is necessary to provide a logic device for deciding to trip the automatic protection switch on the basis of all the FERF signals supplied by these SDH terminals, and to provide a suitable interface for controlling the automatic protection switch in the submarine link terminal equipment. The SDH send terminals available off the shelf do not include these devices and therefore cannot be used without modification.
The submarine link terminal equipment and the SDH terminals being physically separate entities, it is also necessary to interconnect them to transmit the automatic protection switch control signal. This requires the SDH send terminals to be located at a short distance from the submarine link terminal equipment or the provision of regenerators on the link transmitting the automatic protection switch control signals. This device known in itself therefore has the following disadvantages:
The necessity to modify conventional SDH send terminals, available off the shelf but with no automatic protection switch control feature.
The reaction time in the event of failure, i.e. the time to trip the automatic protection switch, depends on the specifications of the SDH send terminals and not only on the standardized characteristics of the link.
The need for an additional link including regenerators if the submarine link terminal equipment is to be located at a relatively great distance from the SDH terminals.
The aim of the invention is to propose a terminal for a bidirectional link with automatic protection switching, in particular a submarine link, that does not have these disadvantages.